(a) Field of the Invention
The present invention relates to an inner shield for a color cathode ray tube, and more particularly, to an inner shield for a color cathode ray tube that effectively blocks external magnetic fields such as the earth's magnetic field to thereby minimize mis-landing of electron beams caused by fluctuations in the external magnetic fields. The present invention also relates to a cathode ray tube including the inner shield.
(b) Description of the Related Art
A color cathode ray tube (CRT) is a display device in which a phosphor screen is scanned by three electron beams emitted from an electron gun to realize specific images. A path of the three electron beams is altered by the earth's magnetic field, which is created by the earth's north and south magnetic poles, to thereby negatively affect purity, raster position, and convergence characteristics of the CRT.
The earth's magnetic field includes a vertical component that is vertical with respect to the earth's surface (earth's vertical magnetic field), and a horizontal component that is horizontal to the earth's surface (earth's horizontal magnetic field). Movement of the electron beams by the earth's horizontal magnetic field may be divided into North-South (N-S) electron beams movement and East-West (E-W) electron beams movement, depending on the direction of the CRT.
That is, with reference to FIG. 1A, N-S movement refers to electron beam movement as a result of the magnetic field (indicated by the arrows) in the vertical direction in the figure (N-S direction) that is parallel to a tube axis Z of the cathode ray tube. Further, with reference to FIG. 1B, E-W movement refers to electron beam movement as a result of the magnetic field (indicated by the arrows) in the horizontal direction in the figure (E-W direction) that is parallel to the screen of the cathode ray tube.
Forces received by the electron beams caused by the earth's magnetic field include a horizontal component and a vertical component. It is mostly the horizontal component that affects picture characteristics of the CRT. This is because with the shadow mask having elongated vertical slots used mainly in consumer CRTs(also referred to as color picture tube) and the shadow mask having dot-shaped holes used mainly in commercial CRTs(also referred to as color display tube), the horizontal component that moves the electron beams in the horizontal direction (x-axis direction) moves the electron beams away from their designated slots or holes.
Therefore, an inner shield is mounted within the CRT to minimize movement of the electron beams caused by the influence of the earth's magnetic field. A conventional inner shield is shown in FIG. 2.
An inner shield 100 causes offset or reinforcing interference with the earth's magnetic field in areas surrounding the path of the electron beams to thereby vary distribution of the earth's magnetic field (in these areas) in directions that minimize changes in the landing of the electron beams. The inner shield 100 is mounted to a mask frame (not shown) and surrounds a path of the electron beams within a funnel (not shown) of the CRT. The inner shield 100 includes an electron gun opening 102 and a screen opening 104 through which the electron beams pass, and a pair of long sections 106 and a pair of short sections 108. The long sections 106 and the short sections 108 are interconnected to form the electron gun opening 102 and the screen opening 104.
Formed at the end of each of the short sections 108 forming the electron gun opening 102, is a V-shaped cutaway section 110 for minimizing N-S electron beams movement. A depth h of the V-shaped cutaway sections 110, which is measured from an apex of the V-shaped cutaway sections 110 to imaginary lines formed flush with edges of the long sections 106 forming the electron gun opening 102, is inversely proportional to an amount of N-S electron beams movement and directly proportional to an amount of E-W electron beams movement. That is, the greater the depth h of the V-shaped cutaway sections 110, the greater the reduction in the amount of N-S electron beams movement and the greater the increase in the amount of E-W electron beams movement.
As a result of this adverse affect on E-W electron beams movement while favorably affecting N-S electron beams movement, the depth h of the V-shaped cutaway sections 110 is limited to a predetermined range. This means that N-S electron beams movement may be controlled only up to a point.
FIG. 3 shows a one-quarter section of a screen, where the x-axis indicates a distance in the horizontal direction from a center 0 of the screen, and the y-axis a distance in the vertical direction from the center 0 of the screen.
With reference to FIG. 3, since the cutaway sections are formed as V-shaped elements in the conventional inner shield 100, although the amount of N-S electron beams movement is effectively reduced in a corner area {circle around (3)}, there is limited reduction in the amount of N-S electron beams movement in the remaining areas of measurement ({circle around (1)}, {circle around (2)}, {circle around (4)}, and {circle around (5)}). In particular, in the area {circle around (4)} that has an insufficient amount of screen margin (also referred to as overspill margin), an effective reduction in N-S electron beams movement is unable to be realized such that the overall quality of the CRT is reduced.
Screen margin refers to the amount of margin that exists before the electron beams land on adjacent phosphors of another color when landing errors occur. Screen margin is affected by such factors as pitch, phosphor width, electron beam size (mask hole size), landing errors, and phosphor arrangement errors.
In a CRT where the mask and screen are formed in a globe-like shape about a line passing through a tube axis of the electron gun, deviation in the emission angle of the electron beams results in identical amounts of deviation over the entire area of the screen. However, in a CRT where the mask and screen are formed in a flat configuration, the amount of deviation on the screen is not identical over the entire area of the screen. That is, for a flat screen, the same deviation in the emission angle of the electron beams translates into larger amounts of deviation on the screen at peripheral areas thereof. It is for this reason that there is an insufficient screen margin in the area {circle around (4)} as described above.